In our studies of the role of the cell surface in the differentiation of the lymphoid system, we have been concentrating on the examination of cell surface markers specific by the T1a region of the mouse (defined as that segment of Chromosome 17 lying distally to the H-2 complex). To date, six different cell surface markers are known to be determined by this region. Four of these markers, Qa-2, Qa-3, Qat-4, and Qat-5 were discovered either in our laboratory or in collaboration with other laboratories. These last two antigens, Qat-4 and Qat-5, were found within the last year. They are represented on subpopulations of peripheral T lymphocytes and are absent from B lymphocytes. Their relationship to differentiation and to other Qa markers is now under investigation.